Sheath heater

ABSTRACT

A sheath heater in a sheathed-type glow plug for diesel engines is described, having at least one generally internal insulation layer and at least one generally external conductive layer, both layers making up a ceramic composite structure. The sheath heater has a generally uniform overall cross-section, generally over its entire length, and, in the area of a tip of the sheath heater, the proportion of the insulation layer in the overall cross-section increases, whereas the proportion of the conductive layer in the overall cross-section decreases.

FIELD OF THE INVENTION

The present invention relates to a sheath heater, especially for use ina sheath-type glow plug for diesel engines.

BACKGROUND INFORMATION

The technology of modern diesel engines places great demands onsheathed-type glow plugs, especially with regard to size, sturdiness,rapidity of heating-up, and resistance to high temperatures. It isusually desirable that, at a heater output of roughly 70 to 100 W, atemperature of 1000° C. and a steady-state temperature of 1200° C. canbe achieved within 2 seconds.

Conventional sheathed-type glow plugs have metallic and ceramic heaters.Customary designs of ceramic sheathed-type glow plugs have internalmetallic or ceramic heaters, which are sintered into a nonconductiveceramic that is stable at high temperatures. However, sheathed-type glowplugs having this type of design can only be manufactured usingexpensive heat pressing methods. On the other hand, sheathed-type glowplugs having external heaters made of composite ceramics can bemanufactured using simpler and more cost-effective sintering methods.

A diesel-engine glow plug having a cylindrical metal tube, a connectingdevice for the electoral contact, and a ceramic heating device, isdescribed in, for example, PCT Application WO 96/27104. In this glowplug, the cylindrical metal tube at its tip supports the ceramic heatingdevice in a floating manner, the ceramic heating device being contactedusing the connecting device, so that during the glow process a currentflows through the ceramic heating device.

In this context, the ceramic heating device has at least one locationhaving a reduced cross-section, the reduction of the cross-section ofthe ceramic heating device occurring at the location at which thefuel-air mixture strikes. The cross-section reduction in this ceramicheating device is realized such that the thickness of the lateral wallis correspondingly reduced at the location in question.

In a sheathed-type glow plug of this type, it is possible that the areaof the heating device that is most accessible to the combustible mixturereaches the necessary ignition temperature the most rapidly due to theresulting greater resistance. As a result, shorter heating-up times arepossible for the sheathed-type glow plugs. A defined reduction of thewall thickness of this magnitude makes it possible to bring to thehighest temperature precisely the location of the sheathed-type glowplug where the combustion mixture strikes.

In PCT Application No. WO 00/35830, a further conventional solution isdescribed for creating a rapidly self-heating sheath heater, achievingthis by reducing the cross-section of the sheath heater in the area ofthe hot zone. A sheath heater of this type, for the purpose ofcross-section reduction, is configured having a filigree tip.

Conventional sheath heaters of this type have the disadvantage that theyhave a hot zone that must be created in an extremely finely fashion byforming a pointed tip or otherwise reducing the cross-section in thearea of the tip of the sheath heater, in order to be able to be heatedrapidly to a high temperature.

However, filigree tips of sheath heaters, that are therefore onlycapable of standing up to small stresses, are extremely sensitive andcan be easily damaged, especially during handling, installation in theengine, etc.

Furthermore, areas of sheath heaters that are reduced in theircross-section in this manner also have an insufficient thermal mass, sothat it is impossible to achieve satisfactory temperature stability, andtherefore in response to a sudden cooling in the environment, such asduring a cold start of the engine, the danger of blowing out thesheathed-type glow plug is very great.

SUMMARY

In accordance with an example embodiment of the present invention, asheath heater in a sheathed-type glow plug for diesel engines may havethe advantage that, as a result of the changed shape of the tip of thesheath heater, it is possible to achieve significantly greatermechanical stability, because the tip of the sheath heater is notreduced in its overall cross-section.

In addition, the heater tip may have a greater thermal mass. This hasthe effect, under certain operating conditions, specifically in a coldstart, of working against a blow-out of the sheathed-type glow plug.

According to one example embodiment of the sheath heater, the latter isconfigured so as to be generally rotationally symmetrical. This may beadvantageous because, as a result of a sheath-heater configuration ofthis type, it is possible that the glow plug glows in its central tiparea, as is required for modern, direct-injection diesel engines.

In this context, in configuring the sheath heater, it can be providedthat the insulation layer is generally surrounded by the conductivelayer.

It has been demonstrated that it is advantageous, especially for theproduction of the sheath heater, if the insulation layer is surroundedby the conductive layer in a generally sandwich-like manner, i.e., ifthe cross-section includes a sequence of conductive layer, a centralinsulation layer, and once again a conductive layer, the insulationlayer being situated at least approximately in a central area of thecross-section of the sheath heater.

It may be advantageous if the sheath heater is manufactured byinjection-molding, and if the insulation layer is injection-moldedfirst, the insulation layer extending, in its edge area, i.e., the areanot bordering on the conductive layer, at least in part right to theperiphery of the sheath heater. As a result, the insulation layer can beplaced in a tool so the conductive layer can be sprayed on, for example,perpendicular to the tool parting plane.

In particular, with regard to the size of the sheath heater, which maybe kept very small, it may be advantageous if the sheath heater has adiameter in the range of roughly 2 mm to 5 mm.

It is expedient if the arrangement of the conductive layer and theinsulation layer is optimized for the specific manufacturing process ofthe sheathed-type glow plugs. Preferred manufacturing processes areinjection molding and/or injection pressing. The optimizationadvantageously takes place using analytic processes, in particular,using a finite-element process. Using an optimization of this type, itis possible to calculate a geometry of the sheath heater which can beproduced very simply and cost-effectively using a two-stageinjection-molding process, without reworking and subsequent sintering.

In this context, it is preferred if the ceramic composite structure ofthe conductive and insulation layers has as constituents tri-silicontetra nitride and a metallic silicide. In this context, it is greatlypreferred if the ceramic composite structure for the conductive layer bemade of 60 wt. % MoSi₂ and 40 wt. % Si₃N₄, as well as sinteringadditives, and for the insulation layer to be made of 40 wt. % MoSi₂ and60 wt. % Si₃N₄, as well as sintering additives.

BRIEF DESCRIPTION OF THE DRAWINGS

Three example embodiments of the sheath heater according to the presentinvention in a sheathed-type glow plug for diesel engines areschematically depicted in the drawing and are discussed in greaterdetail in the description below.

FIG. 1 depicts a longitudinal cutaway view of a sheath heater, havingtwo associated cross-sections, along the lines A—A and B—B, inaccordance with a first example embodiment of the present invention.

FIG. 2 depicts a conductive layer, optimized using a finite-elementcalculation, of a tip area of a sheath heater according to a secondexample embodiment.

FIG. 3 depicts the insulation layer that is associated with theconductive layer depicted in FIG. 2.

FIG. 4 depicts a three-dimensional representation of a sheath heateraccording to FIGS. 2 and 3.

FIG. 5 depicts a view from the rear of the sheath heater according tothe embodiment depicted in FIGS. 2 through 4.

FIGS. 6a) through c) depict a cross-section, a longitudinal cutawayview, as well as a top view of a sheath heater according to a thirdexample embodiment of the present invention.

DETAILED DESCRIPTION

In FIG. 1, a sheath heater 1 is depicted in a longitudinal cutaway view,a conductive layer 2 being generally external and an insulation layer 3being generally internal, insulation layer 3 being surrounded byconductive layer 2 in a sandwich-like manner. Both layers 2, 3constitute a ceramic composite structure.

This sheath heater 1, as can be seen in FIG. 1, has a uniform overallcross-section over its entire length, insulation layer 3 in the area ofa tip 4 of sheath heater 1 undergoing a cross-sectional expansion,whereas the portion of external conductive layer 2 is correspondinglyreduced in comparison to the overall cross-section.

As can be seen, in particular, from the appropriate cross-sections alongthe lines A—A and B—B in FIG. 1, the sheath heater according to theexample embodiment is configured in a symmetrical fashion. Symmetrical,in this context, can denote a symmetry about an axis of symmetry lyingin the cross-sectional plane, or a symmetry about a rotational axisalong the axis of the sheath heater in a crystallographic sense.

A ceramic sheath heater 1 having an external heater has a diametersuitable for installation in an M8 housing. For this purpose, a diameterof roughly 3.3 mm may be advantageous for sheath heater 1.

By appropriately selecting the geometry of conductive layer 2 and ofinsulation layer 3, as depicted in FIG. 1, it is possible to reduce thecross-section of conductive layer 2 in tip area 4, entire sheath heater1 having generally one uniform cross-section over its entire length. Inthis manner, it is possible for sheath heater 1 to glow rapidly in tiparea 4, as is required for modern, direct-injection diesel engines,while nevertheless having good mechanical stability.

In FIGS. 2 through 5, in which for reasons of clarity the same referencenumerals for functionally equivalent components are used as in FIG. 1, asheath heater 1 is depicted, whose shape, more specifically the shape ofconductive layer 2 with respect to insulation layer 3, has beenoptimized using an analytic method, the optimization being carried outwith reference to the manufacturing process of sheath heater 1,specifically with regard to an injection-molding process.

A sheath heater 1 of this type can be realized using a simpleinjection-molding process, insulation layer 3 being pre-injected in apre-shaped tool, and ceramic conductive layer 2 being injected aroundinsulation layer 3 in a second working step.

An expansion 3A, depicted in FIGS. 2 to 5, of insulation layer 3 at theedges of sheath heater 1 increases the injection-molding capacity ofsheath heater 1 of this type as well as the positional stability ofinsulation layer 3 in the tool for injecting conductive layer 2. In thisway, an injection-molding of sheath heater 1 is possible withoutmaterial residues, which complicate the aftertreatments.

In accordance with the depicted second exemplary embodiment forcomposite ceramics, for example, using Si₃N₄ and MoSi₂ the geometry isoptimized. In this context, conductive layer 2 is made up at leastroughly of 60 wt. % MoSi₂, 40 wt. % Si₃N₄, as well as sinteringadditives, and insulation layer 3 is made up of 40 wt. % MoSi₂, 60 wt. %Si₃N₄, and sintering additives.

To produce the injection-molding masses, the powder mixtures are mixedtogether with a polypropylene that is treated using acrylic acid ormaleic acid anhydride, such as polybond 1000 binders and cyclododecane,or cyclododecanol as auxiliary materials, which have a total proportionof 15 to 20 wt. % of the injection-molding mass.

In FIGS. 6a) through c), a sheath heater 1 that is even furtheroptimized with respect to its manufacturing process is depicted in across-sectional cutaway view (FIG. 6a), in a longitudinal section (FIG.6b), as well as in a top view (FIG. 6c)

In this context, the transitions between insulation layer 3 andconductive layer 2 have been rounded, or rounded off, which also may beadvantageous with regard to the injection-molding, because afterconductive layer 2 is sprayed on, no spikes of thermal stresses occur atsharp edges and corners.

In the cross-sectional representation of FIG. 6a, once again the shapeof sheath heater 1, which is optimized with respect to theaforementioned material and the injection method, can be seen moreclearly as a result of exemplary size specifications. In this context,diameter d1 of the sheath heater is 3.3 mm, width b1 of insulation layer3, between the shoulders, is 1.9 mm to 2 mm, the thickness, i.e., thediameter, of heating channel d2 is 0.35 mm, and the thickness of theinsulation layer is 0.8 mm. Advantageously, angle α of theinsulation-layer shoulder is 120°.

Sheath heater 1, depicted in FIG. 6, is also generally a sheath heater 1having a sandwich-like design, in which insulation layer 3 is disposedgenerally between conductive layers 2, insulation layer 3 running atleast partially up to the edge of sheath heater 1.

By way of example, the sequence of the injection-molding of a sheathheater is briefly explained below.

In a first segment, insulation layer 3 is injection-molded. In thiscontext, the first view is at the thickest point of insulation layer 3,i.e., in accordance with the present invention, it is in the area of tip4. Assuming a length of conductive layer 2 of roughly 50 mm, it iscurrently possible in a metallic tool to injection-mold a layerthickness of a minimum of 0.8 mm. If a heat insulating layer is appliedto the surface of the cavity of the injection-molding tool, such asAl₂O₃, ZrO₃, or the like, then even thinner insulation layers 3 can beinjection-molded.

Subsequently, this insulation layer 3 is placed in the toolperpendicular to the tool parting plane, i.e., standing up, andconductive layer 2 is sprayed on.

In this context, the spraying takes place at the foot, the spraying-overof insulation layer 3 using conductive material takes place from thefoot to tip 4. In this context, the surface of insulation layer 3 meltsin a short time and binds to conductive layer 2. The contour ofinsulation layer 3 at the tool wall is configured so as to have fouredges, so that these edges can easily be reached by the melted mass ofthe conductive layer, i.e., can be fused. The rounded-off transitionsare especially provided for this purpose.

On the other hand, if insulation layer 3 and conductive layer 2 are notdesigned to melt immediately in the area of the surface of the cavity,then the tool surface can once again be provided with a heat insulatinglayer in the area of the transition of insulation layer 3 and conductivelayer 2.

Subsequently, the material mass of the conductive layer is machined offat the foot up to the beginning of insulation layer 3, so that the footarea is not electrically short-circuited. A thermal release and asintering then follows.

What is claimed is:
 1. A sheath heater in a sheathed-type glow plug fora diesel engine, comprising: at least one generally internal insulationlayer; and at least one generally external conductive layer, the atleast one generally internal insulation layer and the at least onegenerally external conductive layer together forming a ceramic compositestructure; wherein the sheath heater has a uniform overall cross-sectionalong an entire length of the sheath heater, and, in an area of a tip ofthe sheath heater, a proportion of the insulation layer in an overallcross-section increases relative to a remaining portion of the sheathheater, and a proportion of the conductive layer in the overallcross-section decreases relative to the remaining portion of the sheathheater.
 2. The sheath heater as recited in claim 1, wherein thecross-section is configured so as to be generally symmetrical.
 3. Thesheath heater as recited in claim 1, wherein the insulation layergenerally surrounded by the conductive layer.
 4. The sheath heater asrecited in claim 1, wherein the insulation layer is surrounded by theconductive layer in a sandwich-like manner.
 5. The sheath heater asrecited claim 1, wherein the sheath heater has an overall diameter in arange of 2 mm to 5 mm.
 6. The sheath heater as recited in claim 1,wherein a shape of the conductive layer and of the insulation layer withrespect to each other is optimized using a manufacturing process.
 7. Thesheath heater as recited in claim 6, wherein the optimization is carriedout using an analytic method.
 8. The sheath heater as recited in claim7, wherein the analytic method is a finite-element method.
 9. The sheathheater as recited in claim 8, wherein the finite-element method issupplemented by a statistical evaluation method.
 10. The sheath heateras recited in claim 1, wherein the sheath heater is manufactured usingat least one of an injection-molding method and injection-pressingmethod.
 11. The sheath heater as recited in claim 1, wherein the ceramiccomposite structure has as constituents tri-silicon tetra nitride and ametallic silicide.
 12. The sheath heater as recited in claim 11, whereinthe conductive layer is made of 60 wt. % MoSi₂, 40 wt. % Si₃N₄, andsintering additives, and the insulation layer is made of 40 wt. % MoSi₂,60 wt. % Si₃N₄, and sintering additives.
 13. The sheath heater asrecited in claim 1, wherein the ceramic composite structure is formedbased on a SiOC-glass ceramic derived from polysiloxane and havingsillers and a metallic silicide.